Filter circuits



B. TREVOR FILTER CIRCUITS Filed Feb. 19, 1938 April 29, 1941.

2 Smets-Sheet 2 l l 25% 26 25, L/ 1.9 26 (43 L49 .l 007' 00T/ INVENTOR. femm rREvoR BY M2M ATTORNEY.

Patented Apr. 29, 1941 FILTER CIRCUITS Bertram Trevor, Riverhead, N. Y., assignor to Radio Corporation of America, a. corporation of Delaware Application February 19, 1938, Serial No. 191,424

22 Claims.

The present invention relates to electrical wave filter circuits, and particularly to filter circuits whose individual tuned circuits are comprised of coupled concentric line resonators.

Heretofore, in electrical wave band pass lters employing a pair of coupled tuned circuit-s, a particular diihculty often experienced has been due to the considerable inductanct possessed by the leads between the tuned circuits of the filter. The induct'ance of these leads, it is known, disturbs the normal operation of two coupled cir cuits in a band pass filter.

Among the objects of the present invention are: To provide -a band pass filter wherein the length of the coupling lead or leads between tuned circuits of the `filter are a minimum; to provide an exceptionally compact filter unit occupying small spa-ce; to provide a compact lter unit which is inherently completely shielded; to provide a filter unit which is very stable mechanically and electrically; to providea band pass filter capable of obtaining'a fiat pass band of less than one percent of the mid-band frequency; and to provide a filter unit especially adaptable to ultra high frequencies above fty megacycles.

In general, the filter of the present invention utilizes two or more coupled tuned circuits, each of which consists of a concentric line resonator. In the preferred embodiments of the invention, one concentric line resonator is confined within another concentric line resonator, thus minimizing the length lcf the coupling lead or leads between resonators and also providing a Very compact lter unit which is inherently completely shielded, among other things.

As is known, a concentric line resonator `comprises an outer conductor and a coaxial inner conductor, both conductors being coupled in such fashion that the inductance of the conductors together with the capacitance between same combine to form a resonant circuit whose resonant frequency is determined to a large extent by the dimensions of the conductors. One such concentric resonant line consists of two concentric conductors conductively coupled together at one of their adjacent ends and capacitively coupled together at 'their other ends. Examples of such concentric line resonators are described in British Patent No. 460,118, dated January 21,

1937; United States Patent No. 2,102,805, granted .in conjunction with drawings, wherein like reference numerals represent like parts Athroughout the figures. In these drawings:

Figs. 1, 2" and 3 illustrate, in section, examples of band pass filters made up of concentric line resonators, in yaccordance with the principles of the present invention;

Figs. la and 3a. are the equivalent electrical circuits for the band pass filters of Figs. 1, 2 and 3, respectively,

Fig. lb is the electrically equivalent circuit of Figs. 1 and 2 when the coupling capacitor between the tuned circuits has a particular value;

Fig. 3b is the electrically equivalent circuit of Fig. 3 when the coupling capacitor between the tuned circuits has a particular value;

Fig.V 4 and Fig. 5 show practical mechanical constructions of the band pass filters of Figs. 1 and 3, respectively;

Figs. 6 and 7 are further modifications of the filters of Figs. l and 2; and

Figs. 8 and 9 illustrate .band pass filters in accordance with the invention, which are suitable for push-pull circuits.

Referring to Fig. 1, there is shown a band pass filter composed of .two concentric line resonators, one within the other. The terminals of the filter are designated I, 2 and 3, 4. The outer resonator is comprised of two concentric, electrically conductive, cylindrical surfaces 5 and 6 which are directly connected together at one end by a Inetallic end plate I and capacitively coupled together at their other ends by means of the spacing between metallic plates 3 and 9. Plate 9 is directly connected to one end of the conductor or conducting surface 6, while plate 8 is connected to the adjacent end of the outer conductor or conducting surface 5. The inner resonator is comprised of two concentric, electrically conductive surfaces I0 and II which are directly connected together at one'end by plate 9 land capacitively coupled together at their other ends by means of the spacing between metallic plates I2 land 1. Plate I2 is connected to the end of the electrically conducting surface IIl.` The capacity coupling between plates 8 and 9 and between plates 'I and I2 may be adjusted to provide the desired resonant frequencies for the two tuned resonator circuits. It is preferred that the electrically conducting surfaces 5, 6 and II form parts of hollow copper 'cylinders made up of imperforate material, in which case surface 6 will be the outer surface and surface II the inner surface of a single cylinder.

The two tuned concentric line resonators '5, 6, 9, 8 and II, I0, I2, 'I are designed to have frequency characteristic curves which give a band pass lter section when each is properly adjusted and both coupled together. The two concentric line resonators are coupled together by a suitably selected variable capacitor S1, preferably located near the middle of the two cylinders, this capaci- .conductors in the lter circuit are determined by the conductor diameters, the mid-band frequency, the image impedance, and the band width. The inner and outer resonant circuits are each Ytuned to the upper nominal cut-off frequency.

the desired resonant frequency for various lengths and diameters of conductors, since a long inner conductor has high inductance, and a small diameter inner conductor gives high inductance. If the band width and mid-band Vfrequency of this type of filter are fixed, the image impedance becomes higher as higher inductance conductors are used. Higher inductanee conductors naturally require smaller values of tuning capacitance.

Again, if the mid-band frequency and image impedance are fixed, the conductor inductances will have to be increased as the band width is increased. In actual practice, there is a limit to the minimum value of tuning capacitance that Vcan be used, and also the image impedance cannot be greater than the resonant impedance of one of the circuits alone. These two facts limit the choice of conductor lengths and diameters.

Fig. 2 shows an alternative arrangment, equivalent to that of Fig. 1. In Fig. 2 the two concentric line resonators are shown side by side, in-

stead of being one within the other, as illustrated in Fig. 1. In Fig. 2 the elements which are located in similar arrangement to the corresponding elements of Fig. 1 are labeled similarly but with a prime designation.

A better understanding of the systems of Figs. 1 and 2 may be had by referring to their equivalent electrical circuit shown in Fig. 1a. In Figs. 1, 2 and 1a, the same reference designations have been used to designate equivalent elements.

Fig. 1b is electrically equivalent to Figs. l, 2 and 3 when the value of the capacitance of S1 is equal to 4C1, assuming that the capacitor Si is connected to the middle of 21,2 in each case.

It will be observed that the circuit of Fig. 1b is described by Shea in his book "Transmission Networks and Wave Filters, published in New York by D. Van Nostrand Co., Inc., 1929, on page 316. From the data given in this book, the following relations immediately follow which are applicable to the circuits of Figs. 1, 2 and 1a; assuming that capacitor S1 is connected to the middle of Zllain each case.

It is obvious that there exists a large range of values of tuning capacitance to give 7 where S1, Lz and C2 are as indicated in Figs. 1, 2 and 1a; f1 and f2 are the lower and upper cut off frequencies respectively; R is the impedance that will terminate the filter without'relection at the mid-frequency fm; and Zr is the image impedance, or that impedance which will terminate the filter at any frequency f within the pass band without reflection occurring at the terminal. These formulas then give information in concise form for the design of Ythis type of band pass filter. The lengths and diameter ratios oi the concentric line sections may be varied as desired to obtain the desired inductance for any particular design.

It should be pointed out that the iilter of Fig. 1 may be constructed with unequal values of the capacitance Cz/z with a corresponding inequality of inductances Il! and 6 to give a filter section having unequal terminal impedances. The formulas given above must be altered accordingly to give the proper values of iilter elements.

It should also be noted that the nature of the concentric lines used is such that no coupling exists between the inner and outer resonators, unless the capacitance S1 is provided as shown in Figs. 1 and 2, and S in Fig. 3. This capacitance need be no more than a hole in conductor Il for small values of coupling.

A simple modication of Fig. 1 gives the high pass filter of Fig. 3. In Fig. 3 the capaoitances Cz/ of Figs. 1 and 2 have been removed, thus destroying the band pass action of the iilter and giving a high pass type. Fig. 3a shows the electrical circuit equivalent Vto the circuit of Fig. 3.

Fig. 3b shows an electrical circuit equivalent to Figs. 3 and 3a where S=4C, assuming that the capacitance S is connected to ZL at the middle of each case.

It is to be understood that the capacitance S of Figs. 3 and 3a, or S1 of Figs. 1, 2 and 1a, may be connected to the in-ductors 2L in the first case and 2L2 in the second case at any points along their lengths rather than at the middle. In the former situationl the formulas given above would be altered to meet this new condition.

An inspection of Fig. 3b shows it to be the midshunt section of the constant-7c type of high passk filter described by Shea on page 306 of his boiok, supra. From the relations given by Shea, the following relations immediately follow which are applicabile to the circuits of Figs. 5 and 6.

, L 21m/TS' Z R v 1 (fc/f)2 where L and S are as indicated in Figs. 3 and 3a, R is the impedance that will terminate the filter without reflection as the frequency f approaches infinity; ZI is the image impedance at any frequency f, above fe, which will terminate the :tilter without reflection .occurring at the terminal; and fe :is the cut oi frequency below which the filter gives attenuation. It is of course assumed that we are dealing with frequencies lower than those at which the conductors 2L resonate with their own distributed capacitances, as above these frequencies the conductors would, in effect, change from inductive to capacitive reactance.

This assumption applies as well to high pass filters employing coiled inductances.

-This frequency at which the shunt inductors resadjusted to enable the lter to givev a flat response over the pass band. The system of Fig. 4

lends itself to the use of very small coupling capacita-nces S1, which may be formed by screwing a variabile length rod I3 into :cylinder I0 through the aperture in cylinder II, 6. A covered hole may be conveniently placed in cylin- -f der 5 to allo-W access to this rod I3. For wider pass bands, a small variable capacitor may be used having an external adjustment such as a tuning dial or a screw driver hole.

In Fig. 5, conventional variable capacitators C1 and C2 are employed to tune the two reso-nant circuits. These capacitors would normally be controlled externally. The following approximate dimensions, given by way of example only, were employed in the construction of the model of Fig. 5:

Length of cylindrical conductor 6, 11:11.0

Length of -cylindrical conductor =12.5" Diameter of cylindrical surface 5: 6.0" Diameter of cylindrical surface 6: 1.67 Diameter of cylindrical surface 10= 0.62 Diameter of cylindrical surface 11: 1.50"

One adjustment of the filter construction of Fig. 5 gave a lter characteristic having a band width of 185 kilocycles at a mid-band frequency of 83.7 megacycles. The tuning capacitance corresponding to Cz was approximately 40 mircromicrofarads and the coupling capacitor S1 was estimated to be .35micno-microfarad. There was no convenient way of measuring C1 but it was estimated to be about 30 micro-microfarads. In this case, the proper damping was obtained in the loss in the filter elements, so that the external resistors R1 and R2 were not used. sli'ould be noted that the foregoing pass band was the smallest pass band obtainable with this particular model.

Another adjustment of the model` of Fig. 5

gave a band width of 5 megacycles at a mid-band frequency of 78.5 megacycles. The coupling capacitance in this case was 15 micro-microfarads and the values of R1 and Rz were approximately 1000 ohms. The values bf C1 and C2 were nearly the same as before.

For ordinary use, the type of 'filter illustrated in Fig. 5 would probably be designed for a band pass of not less than 500 kilocycles at 100 megacycles.

In Fig, 6, the coupling capacitance between the two concentric line resonators is formed by cutting a plurality of slots I4, I4 in the coaxial conductor 6" of the outer resonator. A sliding sleeve I5 is provided to adjust the size of the exp'osed portion of the slots, which in effect varies the coupling capacitance. Such an arrangement may be used only for narrow band widths, inasmuch as the coupling capacitance obtained between resonators in this way is small.

Fig.v 7 shows the filter of the invention with y the `inner coaxial conductor :of the inner resonator reversed in position; that is, turned end to end from the position shown in Figs. '1, 4 and 6. The coupling capacitance Si in this case need not be near the middle but can be at the left hand end of the lter, or may be formed by providing a hole of the correctsize in the end plate I6. 'The input to the filter is shown to Ibe a low impedance coupling loop I1. The output requires a damping resistor R3.

Figs. 8 and 9 show push-pull filter arrangements. In these iilter arrangements, the coupling capacitance between the two concentric line resonators is formed by the two condensers 28 and 29, which are substantially symmetric-ally disposed on opposite sides of the plates 20, 2I, although the coupling capacitance can be formed in a number of other ways. The inner concentric line resonator in these two figures comprises the two coaxial rods I8 and I9 which are capacitively `coupled together .by means of spaced metallic end plates 20 and 2|, together with the inner surface of metallic cylinder 22. The outter concentric line resonator is formed by the outer cylinder 5 and the adjacent surface of cylinder 22. Plates 23, 24 capacitively couple the cylinder 22 to the adjacent ends 25 and 26 respectively of the outer cylinder 5. A suitable conductor 21 supports the cylinder 22 in suitable spaced relationfrom the outer cylinder 5.

It should be noted that the inner concentric line resonator in Figs. 8 and 9 resembles the type of line resonators described in United States Patents Nos. 2.104,915 and 2,104,916, granted January 11, 1938, tio Evans and Thompson, respectively.

It is to be distinctly understood that the filter circuit of the present invention is not limited to the precise arrangements shown, since various modifications can be made therein without departing from the spirit and scope of the appended claims. For example, the conducting surfaces of the resonators need not be cylindrical but may be square, hexagonal, etc.

What is claimed is:

1. A lter circuit comprising an inner and an outer tuned circuit, each in the form of a concentric line resonator, each resonator having an outer and an inner conductor, the inner conductor of said outer resonator being hollow, said inner resonator being located within said hollow conductor, whereby the inner surface of said hollow conductor forms the outer conductor of said inner resonator, and means for capacitively coupling said resonators together in the interior of said outer resonator and intermediate the ends of said resonators to constitute a band pass filter.

2. A filter in accordance with claim 1, characterized in this that said means comprises a capacitor located substantially in the middle of the -longitudinal lengths of said conductors. f

3. A filter in accordance with claim 1, characterized in this that said means comprises a pair of capacitors located` substantially symmetrically on opposite sides of the center of the longitudinal lengths of said conductors. Y

4. A filter circuit comprising a pair of inner and outer tuned circuits in the form of concentric line resonators, each having an outer and an inner conductor, the inner conductor of said outer resonator being hollow, said inner resonater being located within said hollow conductor, whereby the inner surface of said hollow conductor forms the outer conductor of sai'd in"- `ner resonator, the inner conductor of said inner resonator comprising two spaced coaxial rods capacitively coupled together at their adjacent ends, and means for coupling said resonators together. Y

5. An inherently completely shielded electrical wave lter comprising a pair of coupled tuned circuits, each in the form of a concentric line resonator having an outer and an inner conducting surface, the outer conducting surface of each resonator substantially completely enclosing the inner conducting surface thereof, one of said resonators being located within the other, said tuned circuits being electrically coupled together in the interior of said outer resonator to constitute a band pass lter, input terminals for one tuned circuit and output terminals for the other tuned circuit.

6. A band pass filter comprising a first tuned circuit having an outer metallic, substantially cylindrical surface and a smaller inner hollow metallic substantially cylindrical surface, said two surfaces being directly connected together at one of their adjacent ends and capacitively coupled together at their outer ends, a conductor located within and extending substantially the length of said inner surface, said conductor being directly connected at one end to said inner surface and capacitively coupled at its other end to said inner surface, whereby the interior of said inner surface and said conductor form a second tuned circuit, and means for coupling said tuned circuits together. 1

'7. A band pass lter comprising a first tuned circuit having an outer metallic substantially cylindrical surface and a Smaller inner hollow metallic substantially cylindrical surface, said two surfaces being directly connected together at one of their adjacent ends and capacitively coupled together at theirother ends, a conductor located within and extending substantially the length of said inner surface, said conductor being directly connected at one end to said inner surface and capacitively coupled at its other end to said inner surface, whereby the interior of said inner surface and said conductor form a second tuned circuit, and means for coupling said tuned circuits together, said means including at least one slot in said inner surface enabling capacitive coupling between said inner conductor of said second tuned circuit and said outer surface of said first tuned circuit.

8. A push-pull lter circuit comprising a pair of inner and outer tuned circuits in the form of concentric line resonators, each tuned circuit having an outer and an inner conductor, the inner conductor of said outer line resonator being hollow and containing therein said inner resonator, whereby the inner surface of said hollow conductor forms the outer conductor of said inner resonator, said hollow conductor having its ends spaced from and respectively capacitively coupled to the adjacent ends of its associated outer conductor, the inner conductor of said inner resonator comprising two spaced coaxial rods capacitively coupled together at their adjacent ends, and means for coupling said resonators together, said adjacent ends of said Vtwo spaced rods forming one pair of terminals for said filter, the ends of said hollow conductor forming the other pair of terminals for said filter.

9. An inherently completely shielded electrical wave lter comprising a pair of tuned circuits, each in the form of a concentric line resonator having an outer and an inner conducting surface, the outer conducting surface of each resonator' substantially completely enclosing the inner conducting surface thereof, one of said resonators being located within the inner conducting surface of the other, and adjustable means electrically coupling together the inner conducting sur face of one resonator and a conducting surface of the other resonator.

10. A lter circuit comprising a pair of tuned circuits in theV form of concentric line resonators, each having an outer and a coaxial inner conducting surface, the outer conducting surface of one resonator enclosing the other resonator, and a variable 'reactance located within and electrically coupling together the outer conducting surface of said one resonator intermediate its ends with a conducting surface of the other resonator intermediate its ends. 1. Y`

11. An inherently completely shielded electrical wave filter comprising a pair of coupled tuned circuits, each in the form of a concentric line resonator having an outer and an inner conducting surface, the outer conducting surface of each resonator substantially completely enclosing the inner conducting surface thereof, one of said resonators being located within the inner conducting surface of the other, and variable means enclosed within said outer resonator for capacitively coupling the inner 'conducting surface of the inner resonator to the outer conducting surface of the outer resonator.

12. An inherently completely shielded filter comprising a pair of coupled tuned circuits, each in the form of a concentric line resonator having an outer and an inner conducting surface, the outer conducting surface of each resonator substantially completely enclosing the inner conducting surface thereof, one of said resonators being located within the inner conducting surface of the other, and variable means for capacitively coupling the inner conducting surface of the inner resonator to the outer conducting surface lof the outer resonator, said variable means in cluding a plurality of slots around the circumference of the inner conducting surface of the outer resonator and a sleeve for adjusting the eective area of said slots.

13. A filter circuit comprising a pair of inner and outer tuned circuits in the form of concentric line resonators, each having an outer and an inner conductor, the inner conductor of said cuter resonator being hollow and capacitively coupled at both ends to the cuter conductor of the outer resonator, said inner resonator being located Within said hollow conductor, whereby the inner surface of said hollow conductor forms the outer conductor of said inner resonator, the inner conductor of said inner resonator comprising two spaced coaxial rods capacitively coupled together at their adjacent ends, and means for coupling said resonators together, the' ends of said hollow conductor forming one pair of terminals and the adjacent ends of said two spaced coaxial rods forming the other pair of terminals for said filter.

14. A lter circuit comprising a rst tuned circuit comprising an outer conducting surface and an inner conducting surface, said last surface comprising two spaced hollow rods extending in the same straight line and capacitively coupled together at their adjacent ends and conductively coupled at their other ends to said outer conducting surface, other tuned circuits located within Vsaid spaced hollow rods, each of said last tuned circuits comprising a conductor located coaxially with respect to its surrounding hollow rod, each of saidv conductors being capacitively coupled at one end and conductively coupled at its other end to its surrounding hollow rod.

15. A band pass lter comprising a rst tuned circuit having an outer metallic, substantially cylindrical surface and a smaller inner hollow metallic substantially cylindrical surface, said two surfaces being directly connected together at one of their adjacent ends and capacitively coupled together at their other ends, a conductor located within and extending substantially the length of said inner surface, said conductor being directly connected at one end to said inner surface and capacitively coupled at its other end to said inner surface, whereby the interior of said inner surface and said conductor form a second tuned circuit, adjustable means at both ends of said outer surface of said first tuned circuit for varying the capacitive coupling between the conductors of both tuned circuits, and means for coupling said tuned circuits together.

16. An inherently completely shielded ultra high frequency band pass lter comprising a pair of tuned circuits, each being substantially completely enclosed and in the form of a pair of surfaces having uniformly distributed inductance and capacitance, one of said tuned circuits being located within the interior of the other tuned circuit in a space substantially free from the electric and magnetic field set up by current in said other tuned circuit, and means for coupling said tuned circuits together in the interior of one of said tuned circuits.

17. A band pass filter comprising a rst tuned` circuit having an outer metallic, substantially cylindrical surface and a smaller inner hollow metallic substantially cylindrical surface, said two surfaces being directly connected together at one of their adjacent ends and capacitively coupled together at their other ends, a conductor located within and extending substantially the length of said inner surface, said conductor being directly connected at one end to said inner surface and capacitively coupled at its other end to said inner surface, whereby the interior of said inner surface and said conductor form a second tuned circuit.

18. A band pass filter comprising a first tuned circuit having an outer metallic, substantially cylindrical surface and a smaller inner hollow metallic substantially cylindrical surface, said two surfaces being directly connected together at one of their adjacent ends and capacitively coupled together at their other ends, a conductor located within and extending substantially the CSQ length of said inner surface, said conductor be- 1 ing directly connected at one end to said inner surface and capacitively coupled at its other end to said inner surface, whereby the interior of said inner surface and said conductor form a second tuned circuit, and means within the outer surface of said rst tuned circuit for electrically coupling said two tuned circuits together.

19. A band pass filter comprising a first tuned circuit having an outer hollow metallic surface and a smaller inner hollow metallic surface, said two surfaces being connected together at one of their adjacent ends and capacitively coupled together at their other ends, a conductor located within and directly connected at one end to said inner surface and capacitively coupled at its other end to said inner surface, whereby the interior of said inner surface and said conductor form a second tuned circuit, and means Within the outer surface of said irst tuned circuit for electrically coupling said two tuned circuits together.

20. A i'llter circuit comprising a pair of tuned circuits in the form of concentric line resonators, the outer conductor of one resonator being in contact over its entire length with the outer conductor of the other resonator, the contacting surfaces of said resonators having an aperture intermediate the ends for enabling capacitive coupling between the inner conductors of said resonators, input terminals for one tuned circuit and output terminals for the other tuned circuit.

21. An electrical wave lter comprising first, second and third concentrically arranged conductors, said first and second conductors forming one tuned circuit, and said second and third conductors forming another tuned circuit, and reactance means within the outermost one of said conductors and intermediate its ends coupling together said two tuned circuits, an input circuit coupled to one tuned circuit and an output circuit coupled to the other tuned circuit.

22. An inherently completely shielded ultra high frequency band pass filter comprising a pair of tuned circuits, each being substantially completely enclosed and in the form of a pair of surfaces having uniformly distributed inductance and capacitance, one of said tuned circuits being located within the interior of the other tuned circuit in a space substantially free from the electric and magnetic eld set up by current in said other tuned circuit, and means for coupling said tuned circuits together.

BERTRAM TREVOR. 

